This invention relates generally to agricultural machines and, in particular, to a swivel hitch for connecting an implement to a tractor.
Farm tractors have a power-take-off (pto) shaft which is coupled by a primary drive line to an input shaft on an implement. When operating some implements such as mower-conditioners, the primary drive line coupling the tractor pto shaft to the implement input shaft is subjected to rather high joint angles in order to achieve the desired maneuverability of the implement. If these high joint angles are not equal, torsional vibration will be transmitted through the primary drive line from the tractor to the implement.
Equal angle hitches as disclosed in U.S. Pat. No. 3,557,892 to Burrough have been used to maintain equal joint angles in the primary drive line and thereby prevent the transmission of torsional vibration. Even when an equal angle hitch is used, it is difficult to maintain equal joint angles in the primary drive line. If the joint angles are not exactly equal, there will be some vibration in the primary drive line when making sharp turns.
Swivel hitches such as disclosed in U.S. Pat. No. 4,738,461 to Stephenson et al include an upper gearbox coupled to the input shaft of the implement and a lower gearbox connected to the pto shaft of the tractor. In order to achieve the desired maneuverability of the implement during normal operation, the lower gearbox rotates relative to the upper gearbox about a generally vertical axis when the tractor makes turns. Although the Stephenson et al swivel hitch overcomes the vibration problem encountered when making sharp turns with the Burrough equal angle hitch, it will not accommodate different speeds of tractor pto shafts without changing the gear ratios of the gearboxes which would be time consuming. U.S. Pat. No. 4,525,987 to Werner et al discloses a swivel hitch having a lower gearbox with two input shafts for connection to tractor pto shafts of different speeds. The Werner et al swivel hitch is complex and expensive.